This invention relates to the purification of a silicon tetrafluoride gas by removal of acidic compounds, carbon monoxide, carbon dioxide, inert compounds and combinations thereof and, more particularly, to the removal of acidic compounds by use of an ion exchange resin, removal of carbon monoxide by use of a catalyst comprising a catalytic metal oxide on or near the surface of an inert substrate, by removal of carbon dioxide by use of an absorption liquid comprising at least one glycol diether, by removal of inert gases by use of a cryogenic distillation process and combinations thereof.
Polycrystalline silicon is a vital component of many commercial products including, for example, integrated circuits and photovoltaic (i.e., solar) cells. Polycrystalline silicon is typically produced by a chemical vapor deposition mechanism in which silicon is deposited from a thermally decomposable silicon compound, typically silane, onto a crystal seed particle in a fluidized bed reactor. Silane may be produced from silicon tetrafluoride by a reaction between silicon tetrafluoride and a metal hydride such as sodium aluminum tetrahydride (NaAlH4).
Silicon tetrafluoride can be produced by a variety of methods, for example, as a gas from fluorosilicic acid, a by-product in the production of phosphate fertilizers. Commercially produced silicon tetrafluoride gas typically contains a number of impurities such as carbon monoxide, carbon dioxide, inert compounds, metal impurities such as boron, phosphorous and calcium compounds and acid compounds such as hydrochloric acid, sulfur dioxide, sulfur trioxide and hydrofluoric acid. These impurities can cause defects and possibly failure of the microelectronic device. Thus a need exists for processes which reduce the impurities from commercially produced silicon tetrafluoride source gas.